NSDateからNSString生成
-(NSString*)getStringByDate:(NSDate*)_date{
    NSDateFormatter*df=[[NSDateFormatter alloc] init];
    NSCalendar*cal=[[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    [df setCalendar:cal];
    [df setDateFormat:@"yyyy/MM/dd HH:mm:ss"];
    NSString* str=[df stringFromDate:_date];
    [cal release];
    [df release];
    return str;
}